Multiple cellular interactions are required for normal hematopoietic cell differentiation. Abnormalities in these interactions may result in bone marrow failure, especially aplastic anemia. Evidence has accumulated that a suppressor T lymphocyte may be the etiologic agent in some cases of bone marrow aplasia. We have completed a study of the effects of plasmapheresis and lymphocyte depletion in combination with cyclophosphamide and prednisone on patients with cyclophoshapmide and prednisone on patients with pure red cell aplasia and aplastic anemia. A study of the effects of anti-thymocyte globulin in chronic and acute cases of aplastic anemia is in progress. Treated patients and those with partially remitted disease are being studied by using in vitro methods that examine the effects of lymphocyte subpopulations on hematopoietic colony growth an enumerate the number of helper and suppressor T lymphocytes.